Boone Tan vs T'au Light Ochre

How close are they?

ΔE 21.5, loose: different enough that you should pick on purpose. T'au Light Ochre is noticeably darker, more saturated than Boone Tan.

Can I use T'au Light Ochre instead of Boone Tan?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 22). T'au Light Ochre is noticeably darker, more saturated than Boone Tan.

Can I use Boone Tan instead of T'au Light Ochre?

The same difference applies the other way round: Boone Tan is noticeably lighter, more muted.

ΔE is the colour difference between our two swatch estimates: under about 6 is close, over about 15 is loose. Real paints also differ in finish, coverage and how they thin, so test before you commit.